Effective absorption coefficient and effective thickness in attenuated total reflection spectroscopy.

Since the introduction of attenuated total reflection (ATR) spectroscopy for the characterization of materials, attempts have been made to relate the measured reflectivity (R) to the absorption coefficient (α) of the absorbing material of interest. The common approach is limited to the low absorption case under the assumption R∼exp(-αde), where de is an effective thickness, which is evaluated for the lossless case. In this Letter, a more detailed derivation leads to R=exp(-ßdp/2), enabling the definition of an ATR-effective absorption coefficient ß and the penetration depth dp of the electric field in the absorbing material. It is found that ß∼4πε2/λ, where ε2 is the imaginary part of the complex dielectric function of the absorbing material, and λ is the wavelength. An alternative formulation is R=exp(-αdef), where def is a generalized effective thickness for arbitrary strength of absorption which reduces to de in the low absorption limit. The experimental data for water, the biopolymer chitosan, and soda-lime glass prove the reliability of the ATR-effective absorption coefficient in the infrared range.

Structural circular birefringence and dichroism quantified by differential decomposition of spectroscopic transmission Mueller matrices from Cetonia aurata.

Transmission Mueller-matrix spectroscopic ellipsometry is applied to the cuticle of the beetle Cetonia aurata in the spectral range 300-1000 nm. The cuticle is optically reciprocal and exhibits circular Bragg filter features for green light. By using differential decomposition of the Mueller matrix, the circular and linear birefringence as well as dichroism of the beetle cuticle are quantified. A maximum value of structural optical activity of 560°/mm is found.

Fano interference in supported gold nanosandwiches with weakly coupled nanodisks.

We studied the far-field optical response of supported gold-silica-gold nanosandwiches using spectroscopic ellipsometry, reflectance and transmittance measurements. Although transmittance data clearly shows that the gold nanodisks in the sandwich structure interact very weakly, oblique reflectance spectra of s- and p-polarized light show clearly asymmetric line-shapes of the Fano type. However, all experimental results are very well described by modeling the gold nanodisks as oblate spheroids and by employing a 2 × 2 scattering matrix formulation of the Fresnel coefficients provided by an island film theory. In particular, the Fano asymmetry can be explained in terms of interference between the scattered waves from the decoupled nanodisks in the spectral range limited by their respective plasmon resonances. We also show that the reflectance and ellipsometry spectra can be described by a three-layer system with uniaxial effective dielectric functions.

Optical response of supported gold nanodisks.

It is shown that the ellipsometric spectra of short range ordered planar arrays of gold nanodisks supported on glass substrates can be described by modeling the nanostructured arrays as uniaxial homogeneous layers with dielectric functions of the Lorentz type. However, appreciable deviations from experimental data are observed in calculated spectra of irradiance measurements. A qualitative and quantitative description of all measured spectra is obtained with a uniaxial effective medium dielectric function in which the nanodisks are modeled as oblate spheroids. Dynamic depolarization factors in the long-wavelength approximation and interaction with the substrate are considered. Similar results are obtained calculating the optical spectra using the island-film theory. Nevertheless, a small in-plane anisotropy and quadrupolar coupling effects reveal a very complex optical response of the nanostructured arrays.

Graded pitch profile for the helicoidal broadband reflector and left-handed circularly polarizing cuticle of the scarab beetle Chrysina chrysargyrea.

The cuticle of the beetle Chrysina chrysargyrea reflects left-handed polarized light in the broad spectral range from 340 to 1000 nm. Interference oscillations in the experimental Mueller-matrix spectroscopic ellipsometry data reveal that transparent materials comprise the cuticle. A spectral analysis of the interference oscillations makes evident that the pitch profile across the cuticle is graded. The graded pitch and effective refractive indices are determined through non-linear regression analysis of the experimental Mueller matrix by using a cuticle model based on twisted biaxial dielectric slices. Non-uniformity in cuticle thickness as well as in pitch profile near the cuticle surface account for depolarizance of the Mueller matrix. Transmission electron microscopy supports the reliability of the results.

Optical characterization of titania thin films produced by the solgel method and doped with co(2+) at different concentrations.

We report some preliminary results on the fabrication and optical characterization of high-refractive-index thin films of titania doped with Co(2+). These films were supported on silica platesthat were chemically activated to attach both phases. The titania films were produced by the solgel method at room temperature and slowly annealed from room temperature to 230 degrees C; their thickness was approximately 600 ?. The optical characterizations were obtained by the use of spectroscopic ellipsometry, where the dielectric function of the material was obtained as a function of the wavelength. Additionally, the ellipsometric function was modeled to obtain the porosity of the films and their thickness.